Some 171 countries inked the deal today, a record number for a new international treaty. About 15 nations, mainly small island states, had already ratified the agreement.

But dozens of other countries were required to take this second step before the pact came into force. UN Secretary-General Ban Ki-moon said: "Paris will shape the lives of all future generations a profound way - it is their future that is at stake."

Speaking at the opening ceremony, he said the planet was experiencing record temperatures: "We are in a race against time I urge all countries to join the agreement at the national level. Today we are signing a new covenant for the future."